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Why are they alike?
Rafa Nadal is 110 percent heart. He is playing with perhaps the best generation of tennis players, and even though he has less natural talent and technique, he compensates with his power, work ethic and winning mentality to be on the top on a consistent basis.
Football has a player like this: Wayne Rooney. Talent-wise he's just not as good as CR7 or Messi. Wayne's not even as talented as Xavi. But he works hard and hates to lose, and like Nadal, is a power player in his own right.
Some might not like the comparison because Rooney never has dominated his sport (and probably never will) like Rafa has, but the comparison between styles, attitude toward the sport and the winning mentality they both have match them perfectly.
Simply put: Rooney would never be considered in the Top 10 of world football without his winning mentality, just like Rafa would probably be owned by Federer, Djokovic and Murray without his.
Other footballers that may be similar to Nadal:
Samuel Eto'o, Dani Alves, Carlos Tevez.